ENH: shm: blockLevel through ray-shooting

Summary

shm uses a blockLevel specification to close small gaps. This is not really useful for small features which might reside inside a single cell. Also it can quite easily give errors if the geometry is not sufficiently resolved enough. In this feature we use a similar ray-shooting method as is used for the gapLevel refinement - opposite rays are shot from each (candidate) cell centre and if both hit a 'blockLevel' surface it sees if the cell is to be deleted.

The disadvantage of the method is if the gap is much bigger than the refinement level - the cells in the middle of the gap are never seen as candidates and will not get removed.

Details of new models (If applicable)

v2206 tutorial mesh/snappyHexMesh/opposite_walls:

With this change:

Risks

(Possible regressions?) (Changes to user inputs?)